Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a pathological expansion of a polyglutamine domain within the protein huntingtin. The precise mechanisms involved in the etiology are still unknown, however there is evidence that impaired mitochondrial function is likely an important factor in HD. A possible clue to the pathogenesis of HD came with the discovery of neuronal intranuclear and cytoplasmic inclusions composed of mutant huntingtin. It has been suggested that Tissue Transglutaminase (tTG) may be a contributing factor to the formation of these aggregates. Tissue TG is a calcium-dependent transamidating enzyme that catalyzed the formation of isopeptide bonds between specific proteins to produce insoluble polymeric structures. Recently we have demonstrated that TG activity and tTG levels are significantly increased in specific brain regions affected by the disease as compared to control cases. Further, studies from the laboratory demonstrated that tTG associates with a truncated huntingtin protein and activation of tTG resulted in the modification of specific proteins associated only with the mutant truncated huntingtin protein. These results suggest that tTG may have a role in the etiology of HD. Our working hypothesis is that impaired mitochondrial function results in an increase in tTG activity and this subsequently results in an increased association of tTG with truncated huntingtin and modification of specific mutant huntingtin associated protein. In this proposal the majority of the experiments will be carried out in human neuroblastoma cells that express in a stable or inducible manner, physiological or pathological huntingtin protein constructs. In this proposal we will: (1) test the hypothesis that mitochondria impairment results in an increase in TG activity, and that the presence of mutant huntingtin potentiates this response, (2) test the hypothesis that tTG interacts selectively with the N-terminal truncated huntingtin protein, and that mitochondria impairment results in an increase in this interaction, (3) test the hypothesis that tTG contributes to the formation and/or stabilization of the aggregates, and that mitochondrial impairment may potentiate this effect, and (4) identify the mutant huntingtin-associated proteins that are tTG substrates and test the hypothesis that modification of these proteins may contribute to the selective neuronal death in HD.